2() YOUNG TWIN HUMAN BMBEYOS WITH 17 I!) PAIRED SOMITES. 



surface of the notochord and the under surface of the medullary groove in the median 

 plane. There is now no lumen either in this neurenteric canal or in this portion of the 



notochord, it having been obliterated by the crowded, densely packed cells forming these 

 structures. There is no doubt of this representing the neurenteric canal, as the structure 

 is perfectly evident and distinct, and there is no solid tissue adjoining, with which it might 

 be confused. The notochord extends caudad of the neurenteric canal for a distance of 

 8 sect ions (80m) before fusing with the primitive streak. This is in accord with the findings 

 of Eternod, for the notochord in his embryo of 2.1 1 mm. also passed caudad to the canal. 

 In his case the only difference was the possession of a lumen by the structures, and thus of 

 communication with the underlying gut. No communication with the gut occurs in 

 Embryo VI, owing to the presence of the dorsal aorta between it and the notochord. 



The close correspondence of Embryo V to its twin is nowhere better exemplified than 

 in the development of the notochord. The description given above for Embryo VI will 

 serve accurately for the account of the stages seen in Embryo V. It agrees in having the 

 same extent, starting and ending at the same points as in its twin, in having fusions of the 

 same degree with the endoderm and in exactly the same regions, both over the pharynx 

 and over the hindgut. The main difference is that the dorsal aortae are smaller and the 

 areas of their fusions, one over the yolk sac, the other over the cloaca, are less in extent, 

 so that not so long a stretch of the notochord is separated from contact with the endoderm. 

 There is not as distinct an appearance of a lumen in this case, although there is a central 

 space found twice, for two sections in each case, one just behind the pharyngeal region, one 

 where the notochord becomes free from the gut over the cloaca. The neurenteric canal is 

 equally as distinct here (plate 2, fig. 12) as in the other embryo and occurs at exactly the 

 same level. In this case the dorsal aorta ends over the cloaca just before this level is 

 reached and the notochord is very near the gut, and a solid cord of cells connects the noto- 

 chord with the endoderm, exactly in line with the solid cord connecting the notochord with 

 Hi,, medullary groove. The presence of such a distinct neurenteric canal in both of these 

 embryos is a point of especial interest. The notochord ends by fusing with the primitive 

 streak independently of either the nervous system or postanal gut. 



ALIMENTARY SYSTEM. 



The stomodseum here is merely the cleft (plate I) between the head and the heart 

 region. It IS fairly wide and shallow and is closed off from the pharynx by the buccopha- 

 ryngeal membrane. This membrane is perforated by three small apertures, marking the 

 beginning of the rupture and disappearance of the structure. In Low's (DOS) embryo of 

 13 11 somites, which is the nearest younger to this, the buccopharyngeal membrane is 

 complete, while in the nearest older, Van den Brock's ( I'll 1) of 22 somites, only the remains 

 of the membrane arc to be found. In the two embryos last mentioned there is a distinct 

 pouch of Rathke found, but no signs of this pouch are to be distinguished in Embryo VI, 



unless a small Ihickc 1 portion of the ectoderm of the roof of the stomodseum, just in 



front of the buccopharyngeal membrane, be interpreted as such. 



The pharynx (plate )i, figs. 1 and 2), just back of the pharyngeal membrane, is pris- 

 matic, appearing triangular on section, with the apex forming the notochordal groove 

 dorsally. No pouch Of Seessel is recognizable. As the branchial region is reached it widens 

 nut considerably and becomes flattened dorsoventrally. The gill pouches, .'* in number on 



each side, open widely off the cavity of the pharynx, there being practically no constric- 



